Serial interfaces use serial communication protocols to transfer data. Serial interfaces send pieces of information one right after another. In contrast, parallel interfaces send multiple pieces of information simultaneously. In theory, parallel interfaces can send information faster, however they require more lines or wires to transfer the data.
A common serial interface uses only two lines or wires for transferring power and data. Such an interface is used when limited lines or wires are available. For example, automotive systems typically use a two line serial interface for communication. This is due to the typical limited space and wire availability for automotive systems.
However, the common two line serial interface can be too slow for some applications.